Note: Descriptions are shown in the official language in which they were submitted.
0~ 10631
Field of th Invention
~Th~-i~vention relates to an-improved process for~
producing electroIytic manganese dioxide by the electrolysis
of molten manganese nitrate hexahydra~e at a temperature
between about 115C. and about 126C. and with an anodic
:- current density of from 140 to 300 mA/cm .
Back round o~ the Invention
The use of manganese dioxide as an active cathode
material (depolarizer) in dry cells is well known. Manganese
dioxide for cell use can be formed of natural manganese
dioxide ores or it can be electrolytically produced by
electrolyzing a manganous sulfate solution as disclosed in
the publication titled "Batteries" - Vol. l, edited by
Karl V. Kordesch and published by Marcel Dekker, Inc., New
York, 1974. Specifically, the process entails the feeding o~
a preheated M~S04-H2S04 bath into an electrolytic cell which
- is operated with direct current under the following general
conditions:
a) electrolyte concentration - MnsO4, 0.5 to
. 1.2 mole/liter; H2S04, 0.5 to 1.0 mole/liter;
b) electrolyte temperature, 80C. to 100C.; and
c) an anodic current density of 7 to 12 mA/cm2
The anode ma~erial generally employed in this
~ype process is titanium, lead alloy o~ carbon. During
electrolysis, the MnS04 concentration decreases and the
H2S04 concentration increases in the electrolyte with
the net result being that MnO2 is deposited at the anode
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and H2S04 is formed in the electrolyte. The MnO2 is
then removed from the anode and after conventional
post-treatment, it is ready for use as ~n active cathode
material in dry cells.
In Russian Inventor's Certificate No. 379,534
to F. K. Andryushchencko et al published July 59 1973
another electrolytic process is disclosed for the
productlon of electrolytic manganese dioxide which
entails the electrolysis of molten manganese nltrate
hexahydra~e at a t~mperature of 90C. to 105C. and with
an anodic current density of 10 to 15 mA/cm2.
It is an object of the present invention to
provide an improvement in the process disclosed in Russian
Inventor's Certificate No. 379,534 for producing battery grade
MnO~ fr~m electrolyzed ~olten M~(N03)2-6H20.
It is another object to provide a process for
producing electrolytic manganese dioxide from molten manganese
- nitrate hexahydrate that will yield manganese dioxide
equal to or superior to the commercially available mangane6e
dio~ide obtainable from the electrolysis of anaqueous
manganous sulfate solution.
~ till another object is to provide a process
~or producing manganese dioxide from molten nitrate
manganese hexahydrate whereby manganesa dioxide can be
deposited on the anodic electrode at a faster rate, i.e. lO
times or more, than the deposition of manganese dioxide using
the elec~rolytic process of anaqueous manganous sulfate
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solution or the process disclosed in Inventor's Certificate
No. 379,534.
Summary o~ the InventioD
The invention relates to a process for producing
battery grade electrolytic manganese dioxide by elec-
trolyzing molten manganese nitrate hexahydrate
~Mn~N03)2-6H20] at a temperature between about 115C.
and about 126C. and wi~h an anodic current density of
from about 140 ~o about 300 mA/cm . The optimMm con-
ditions for the electrolysis of molten manganese nitratehexahydrate considering current efficiency, quality of
the manganese dioxide deposit, grindability of the
manganese dioxide deposit, and discharge properties
of the manganese dioxide deposit, are to conduct the
electrolysis at a temperature of 117C. + 2C. and with
an anodlc current density of from about 150 to 200 mA/cm2.
In the electrolytic ce11 for use in this invention,
the material of the anodic electrode could be selected
from the group consisting of carbon including graphite,
precoated lead, i.e., precoated with MnO2, for example,
and titanium,with carbon being ~he preferred material. The
cathodic electrode of the cell could be carbon, including
graphit~ or any metallic conductive material preferably
having a low hydrogen overvoltage, such as stainless steel,
platinum, titanium, zirconium or the like.
It has been ~ound that the higher the temperature
at which the electrolysis of the molten manganese nitrate
hexahydrate can be conducted, the better ~he electrochemical
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properties of the manganese dioxide produced. However,
at temperatures above about 126C., the manganese
nitrate hexahydrate begins to thermally decompose as
evidenced by the discharge of brown fumes i.e., N02,
from the electrolytic cell. Thus the h~gh temperature
limi~ation of about 126C. is necessary if thermal decom-
position of the molten nitrate is to be avoided.
However, unexpectedly it has been found that conducting
the electrolytic process at 117C. ~ 2C., the electrochemical
properties of the manganese dioxide produced are optimized
such that its performance as an active cathode material
in a battery is e~ual to or superior ~o that of the best
commercially available manganese dioxide produced by the
electrolysis of an aqueous manganous sulfate solution and
far superior to that of the manganese dioxide produced
in accordance with the teaching of the above-identified
Russian Inventor's Certificate.
It is known that for a constant current
efficiency, the anodic current density is substantially
proportional to the rate of manganese dioxide deposited
at the anodic electrode. Thus, contraty to the prior
art electrolytic processes for producing manganese
dioxide, the process of this invention can be conducted
at 10 times or more the current density of such prior
art processes thereby increasing the deposition rate of
manganese dioxide by a factor-0P 10 or more. This un-
expected high rate of production of mangane~e dioxide in
accordance with this invention can result in either the
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reduction of allocated plant sapce for producing
manganese dioxide, or, using the same plant space, the
manganese dioxide output can be increased by a factor
of 10 or more.
; Brief Description of the Drawin~ Figure 1 is a graph of the closed circuit
voltage vs. milliampere hours for cells using electro-
lytic manganese dioxide of the prior art.
Figure 2 is a graph of the closed circui~ voltage
vs. milliampere hours for cells employing electrolytic
manganese dioxide of the prior art compared to cells
employing electrolytic manganese dioxide made by the
process of the present inventionc
Figure 3 shows a graph of the average cell
voltage vs. time for cells employing the electrolytic
manganese dioxide of the prior art compared to cells--
employing electrolytic manganese dioxide made in accordance
with the process of this inven~ion when discharged across
a 4-ohm load.
Figure 4 shows a graph of the average cell
voltage vs. time for cells employing the electrolytic
manganese dioxide of the prior art compared to cells employ-
ing electrolytic manganese dioxide made in accordance with
the process of this invention when discharge across a
12-ohm load.
Figure 5 shows a graph of the av~rage cell
voltage vsO time for cells employing the electrolytic
manganese dioxide of the prior art compared to cells employ~
ing electrolytic manganese dioxide made in accordance with
~6 ~ ~ 7 ~ 10631
the process of this invention when discharge across a
~ 25-ohm load.
;; The synergistic effect obtained in the pro-
duction of battery grade manganese dioæide from the
elec~rolysis of molten manganese nitrate hexahydrate within
the hlgh temperature range and high anodic current
den~ity range specified above will bec~me apparent
r~m the following exam~les.
EXAMPLE 1
Using the teachings of the prior art (~us~ian
Inventor's Certificate No~ 379,534), manganese dioxide
was produced by electrolyzing manganese ni~ra~e hexa-
hydrate at a temperature of 100C. and with an anodic
current density of 10.6 mA/cm2 in an electrolytic cell
having a pla~inum cathode and a carbon anode. The
manganese dioxide so produced was then blended to produce
a cathode mix having the following proportlons: 0~200
gram Qf MnO2; 2 grams of coke; 1 gram of graphite;
and 0.7 ml 9M KOH electrolyte. A con~entional first
test cell was prepared by placlng inside a plastic con-
tainer the cathode mix having an embedded spiral gold
wire for electrical contact, a separator paper on top
of the mix9 a perfora~ed plastic disc o~ top of the
separator, a 9M KOH electrolyte solu~ion disposed over
the d~sc to fill the container and then a threaded plug
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10631
. . .
containing a frit~Pd glass tube having a platinum wire
counter electrode and an Hg/HgO (9M KOH) reference
electrode was screwed onto the top on the plastic
container thereby securing all the components within
the cell.
The test cell was discharged on a 1 mA conti~uous
drain and the closed circuit voltage vs. the reference
electrode [Hg/HgO (9M KOH)~ was o~served and the data
obtained are shown plotted in Figure 1 as curve A.
A second identical test cell was produced except that
instead of the manganese dioxide used in the first test
cell, the best commercially available grade of electro-
lytic manganese dioxide produced by the electrolysis of
: anaqueous manganous sulfate solution was used, said
manganese dioxide being known commercially as Tekkosha
EMD. The second test cell was then di~charged on a
1 mA continuous drain and the closed circuit voltage
vs. the reference electrode was observed and the data
obtained are shown plotted in Figure 1 as curve B. As
- 20 is apparent from the curves, the Tekkosha EMD was far
superior as an act~ve cathode material than the manganese
dioxide produced by the prior art process of elec~ro-
lyzing molten manganese nitra~e hexahydrate made in
accordance with the teachings of Russian Inventor's
Certificate No. 3799534.
~,
Using an electrolytic cell having a graphite
anode and a platinum cathode, manganPse dioxide was pro-
duced by electrolyzing mol~en manganese nitrate hexahydrate
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at various temperatures and with various anodic
current densities as sho~n in Table I.
TABLE I
De osition Conditions
._ P~
~urren~ Density
Sample# Tempera~ure C. (mA.cm'?
1 105 275
2 105 195
3 117 255
~ 126 318
*5 117 300
*The anodic electrode disintegrated during electrolysis.
Using t~e manganese dioxide samples 1 to 4
and the best commercially available grade of manganese
dioxide produced by electrolyzing an aqueous manganous
sulfa~e solution (Tekkosha), five test cells were produced
- as described in Example 1 such that each cell employed
a different sample of manganese dioxide,
Each of the test cells was then dischared on
a 1 mA continuous drain and the closed circuit voltage vs.
the reference electrode of the test cell was observed.
The data obtained for the tests are shown plotted in
Figure 2 as curves 1 through 5 which correspond to cells
1 to 5 employing the manganese dioxide samples 1 through 4
and the Tekkosha manganese dioxide sample, respectively.
As is apparent from Fi~ure 2, the initial
discharge step was very similar for all cells 1 through
5 with cell 5 (Tekkosha) exhibiting the lowest voltage of
all. Cells 1 and 2, employing the lower temperature
MnO2 materials, dlscharged in the first step at a voltage
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o~ 20 to 50 mV higher than cell 5. Cells 3 and 4,
employing the higher temperature MnO2 materials,
ran about 50 to 100 mV higher than cell 5. It is
in the plateau of the second s~ep where cells l
and 2 showed both low voltage and low capacity. Contrary
to this, cells 3 and 4 were still slightly higher i~
voltage on this pla~eau than cell 5, wi~h ceLl 3, pre-
pared at 117C. and 255 mA/cm2, being at least as good
in capaclty as cell 5 and cell 4 being only slightly
; 10 lower in capacity than cell 5.
On the basis of the chemical testing and the
discharge behavior of the samples of the manganese
dioxide produced, the best material was prod~ced unex-
pectedly at the high temperatures, preferably abou~
117C. + 2C. which is about 10C. below the level at which
some thermal decomposition of the electrolyte can be
observed. Since moderate variation within the current
density range specified above is not as critical as
the temperature variation, then one may choose a value
: 20 just below the limiting current density for the optimum
tem~era~ure to maximize plating rate and current efficiency
while minimizing attack on the anode.
EXAMPLE 3
A total of about lOO g MnO2 was prepared in
five batches by electrolyzing molten Mn(N03)2-6H20 over
the following range of conditions:
Anode - graphite, 23-25 cm2 surface area
Cathode - platinum screen
Temperature - 117C
Average anode zurrent density 140-175 mA/cm2
10 .
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.~ )7~3 10631
Initial anode current density - 175-200 n~/cm2
Current efficiency - 55-83%.
Those runs with current efficiencies less than 80% con-
tained salts with water in excess of the water of
crystallization (-6H20) as evldenced b~ water being observed
c~ming off during the run. Properly dried salts all had
efficiencies greater tha~ 80%.
The ~ive batches were combined, ground in a
glass mortar with a pestle and then air-dried after
being washed in an acid bath. The manganese dioxide
was thereafter used as an ac~ive cathode material in nine
"AA" size ZnC12 test cells. Each cell c~mprised a ~inc
can having therein a coated paper separator liner into
which was placed a cathode mix with a centrally disposed
carbon collector rod, a 32% ZnC12 electrolyte and then
the can was closed using a conventional rim~vent seal.
The cathode mix in each cell weighed 8.3 grams and con-
sisted of 9.18% carbon black; 50.53% MnO2; 12.29 ZnC12
and 28~/o water.
Nine additional cells (control cells) were
produced identical to the cells described above except
that Te~kosha electrolytic manganese dioxide was employed
ins~ead of the manganese dioxide prepared by electrolyzing
molten manganese nitrate hexahydrate.
The 18 cells (nine test cells and nine con~rol
cells) were aged for three weeks and tested for open
circuit voltage (OCV) and flash current (short circuit
current). The data obtained from these tests are shown
in Table II.
11.
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1063 1
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~ 06 9 ~ 7 ~ 10631
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As is apparent from the above, the a~erage
open circuit voltage and average ~lash current of the
cells using the MnO2 produced in accordance with this
invention are substantially equivalent to the average
open circuit voltage and average flash current of the
cells which employed the be~t commercially available
m~nganese dioxide produced by electrolyzing ~n aqueous
manganous sulfate solution.
Three of the test cells and three of the control
cells were then continously discharged across a 4-ohm
load. The data obtained from this test are shown
plotted in Figure 3 with cur~e A representing the test
cells made using the MnO2 as produced in acc~rdance with
this invention and curve B representing the control cells
which used the Tekkosha MnO2. The data for each set of
three cells were plotted for specific time periods and
then a line (vertical line) was drawn connecting the
three points. The midpoints of the vertical lines for
each set of three cells, i.e., the three ~est cells and
the three control cells, were then used in preparing
curves A and B, respectively. As is apparent from ~
Figure 3, the performance of the test cells using the
MnO2 prepared in accordance with the process of this
invention was superior to that of the control cells
which employed Tekkosha MnO2.
Another three of the test cells and another
three of the control cells were continuously discharged ~:
across a 12-oh~ load which represents the typical load :.
of a small calculator. Using the same technique as
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described in conjunc~ion with Figure 3, the data obtained
from this test are shown plotted in Figure 4 with curve A
representing the test cells made using the MnO2 as produced
in accordance with this invention and curve B representing
the control cells which used the Tekkosha MnO2. As is
apparent from Figure 4, the performance of th~ test cells
using ~h~ MhO2 prepared in accordance wi~h ~he process o
this invention was superior to that of the control cells
. which employed Tekkosha ~nO2.
The remaining three test cells and remaining three
control cells were continuously discharged across a 25-ohm
load which represents the typical load of a portable size
radio. Using the same technique as described in conjunction
with Figure 3, the data obtained from this tes~ are shown
plotted in Figure 5 with curve A representing the test
cells and curve B representing the control cells. As is
: apparent fr~m Figure 5, the performance of the test cells
using the MnO2 prepared in accordance with the process
of this invention was superior to that of the control
cells which employed Tekkosha MnO2.
EXAMPLE 4
Twenty alkaline test cells were produced, each
using a zinc anode, a carbon collector rod, a 9N KOH elec-
~rolyte and 3.2 grams o depolarizer mix contain-
ing 80% MnO2 (as prepared and described in Example 3), 7~5/O
graphite, 1.5% acetylene black and 11% 9N KOH.
In addition, 20 identical control cells were produced
except that the MnO2 used was Tekkosha M~O2 prepared by
electrolyzing an aqueous manganous sulfate solu~ion. Five of
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each type of cells were then intermittently discharged
across a different load until a 0.9 cutoff voltage was
observed. The average discharge time for each set of
five cells to a 0.9 volt cutoff and the discharge load
used are shown in Table III. The control cells used in
the 25~ohm, 150-okm and 250-o~m tests were fresh cells
(not aged) and those used in the 125-ohm test were six
months old. All the test cells used in the various ~ests
were four months old.
10 . TABLE III
Intermittent Test Cells Control Cells
Load Discharge Average Discharge Avera~e Discharge
Test Time_ T~me ~hours) Time (hours)
25-Ohm 4 hrsfday11.0 12.4
125-Okm 4 hrs/day65.0 70.0
150-Ohm 16 hrs/day84.0 87.6
250-Ohm 16 hrs/day149.5 143.0
It can be concluded from the above data that the
performance of alkaline cells employing the MnO2 made in
20 accordance with this process is comparable to that of the : :
: ~ alkaline cells employing the best commercially available
MnO2 prepared by electrolyzing an aqueous manganous
., . , ~
sulfa~e solution.
. ' ' .
15.
